This invention relates to a unique pressure relief mechanism that may be useable in different types of devices and typically may be used in devices for dispensing compositions such as caulking materials, grease, automotive windshield sealant, dual component reactive resins, and the like. In a particularly preferred application, the unique pressure relief mechanism is employed in a dispensing device such as a caulking gun. In devices for dispensing such compositions, it is well known to employ a pressure medium driven cylinder/piston unit. Pressure, introduced into the cylinder behind the piston, drives the piston in a forward, linear direction to dispense fluids. Introduction of pressure behind the piston is controlled by actuation of a trigger on the dispensing device. Once the piston has moved its full travel forward, ejecting all of the material from the cartridge, in order to remove the previously emptied cartridge and insert a fresh cartridge into the dispensing device, it is necessary to return the piston. Upon release of the trigger, a pressure relief valve, located within the handle, opens automatically to allow the pressure introduced behind the piston to escape to ambient atmosphere. However, the rate at which this pressure escapes through the relief valve is relatively slow, thus making it difficult for the user to return the piston rapidly and easily to facilitate rapid reloading.
In previously known dispensing devices, in order to return the piston to its initial or returned position upon the completion of a caulking operation, valving structures have been provided to enable the piston to be forced backwards to its initial or returned position by means of air pressure. Hodosh et al. (U.S. Pat. No. 3,768,472) discloses a fluid dispensing gun including an automatic ball-check-valve 150 which permits gas in chamber 44 to freely pass into chamber 42 as the piston is returning, thus permitting relatively rapid movement of the piston as it returns.